It's more "how much grain" than "how many grains", photographic grain is a non-numerable entity, like milk and butter. "Grain" is the visual perception of "clouds" of things that at the microscope look like more or less dense filaments.
Nice microscope pictures here:
The numeric measure of grain is given either by resolution in line-pairs/millimetre, or by RMS granularity.
In the abovementioned document you'll find such measures for many films.
Hope it helps